Null Values in Zones in Highcharts

I have graphs that have zones. Currently if there is a null value, that null value is displayed in the tooltip with the zone color of the point before it. I want the null value to always be the color

For example,

The tooltip in graph 1 has all the correct information... but "Strength: null" has a green dot. I want to make this dot black because it is a null value.

Is it possible to make this happen? Thanks so much.

https://codepen.io/austeng/pen/gOppRWY

zones: [
  {
    value: 0.3,
    color: "green"
  },
  {
    value: 0.5,
    color: "black",
  },
  {
    color: "red"
  }
]